Spin the light

1 · Pick the right light

Beginner

Which pump makes a new signal appear near the Dirac point?

Circularly polarized infrared light Linearly polarized infrared light The ultraviolet probe alone, no pump

2 · Double the field

In the theory, the light-induced mass follows m_F ∝ E₀²/ω³, where E₀ is the amplitude of the pump's electric field and ω its angular frequency. The band gap is 2|m_F|.

Intermediate

With the same infrared light, the field goes from 2,000 to 4,000 kV/cm. By how much is the band gap multiplied?

By 2 By 8 By 4

3 · Read the dip

Near the Dirac point, the measured intensity shows a dip that looks like a band gap. It is far stronger with one probe polarization than with another.

Expert

What do the authors conclude about this dip?

It is the light-induced band gap, finally measured It comes mainly from how the probe pulls electrons out It is an ordinary band gap caused by the substrate

4 · Compare the two declines

Away from the Dirac point, Berry curvature decays as 1/k³ and pseudospin as 1/k, k being the distance in momentum from the Dirac point.

Expert

From a point already far away to one ten times further, how many times more does Berry curvature drop than pseudospin?

10 times 100 times 1,000 times

Sign flipped

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